Lateral organ differentiation and its internal and external control are critical frontiers of developmental plant genetics. Legume nodulation offers an elegant system to study the genetic and biochemical basis of these processes where mutational analysis has coupled structure to function. Legumes form nitrogen-fixing root organs known as nodules after establishment of a symbiotic relationship with rhizobia. Nodule formation is tightly regulated by the plant and can be inhibited by a number of external factors, such as soil acidity. This is of global importance as many of the world’s legume crops, such as soybean, are grown in low pH soils. Despite this, the precise plant mechanism(s) by which low pH conditions inhibit soybean nodule development remains poorly characterised.
We addressed this in soybean using a combination of physiological and genetic approaches. Split-root and grafting studies using an Autoregulation-Of-Nodulation (AON)-deficient mutant line, altered in the AON receptor kinase GmNARK, determined that a systemic, shoot-controlled and GmNARK-dependent mechanism was critical for facilitating the inhibitory effect. Acid-induced inhibition was independent of aluminium ion concentration (AlCl3), and occurred early in nodule development, between 12-96 h post-inoculation with Bradyrhizobium japonicum. Biological effects were confirmed by measuring transcript numbers of known early nodulation genes. Transcripts decreased on both sides of split-root systems, where only one side was subjected to low pH conditions.
To further characterise the genes and regulatory networks involved, a transcriptome analysis via RNA sequencing was undertaken and detected the upregulation of numerous genes involved in oxidative stress, wounding response, metal ion toxicity, and salicylic acid and ethylene biosynthesis and signal transduction, being up-regulated in response to low pH conditions. Our findings enhance the present understanding of the innate mechanisms regulating legume nodulation control under acidic conditions, which could benefit future attempts in agriculture to improve nodule development and biological nitrogen fixation in low pH soils.